1. Field of the Invention
The present invention relates to temperature-controlled spaces, such as environmental chambers and, in particular, to a control system for controlling the temperature of such a space.
2. Description of the Prior Art
It is often necessary to maintain the temperature of an enclosed space at a predetermined temperature or in a predetermined temperature range. Such temperature control is necessary, for example, in the cargo spaces of vehicles, cargo containers, environmental chambers and the like. Such temperature control systems typically include a heating unit and a refrigeration unit and a control system for monitoring the ambient temperature in the served space and comparing it to a set point temperature and controlling the heating and/or cooling output as necessary to maintain the set point temperature.
Various types of control schemes have been used for different applications, but many of these schemes do not provide a very precise temperature control. One type of system simply maintains both the heating unit and the refrigeration unit on all the time and controls the temperature by controlling the mix of heated and cooled air. Other systems leave the heating unit on all the time but control an associated blower to control heated air flow. Such systems are inherently inefficient. Furthermore, systems which control air flow mixtures require dampers and associated ductwork.
A variant type of system maintains the refrigeration system on all the time and controls temperature by turning the heating unit on and off, as necessary. This improves efficiency, but the system is relatively slow to react, since the refrigeration unit must constantly overcome the effect of the heating unit.
Other types of systems control temperature by cycling the refrigeration unit on and off, as needed. This involves the use of cycling solenoid valves which can be relatively noisy in operation and do not provide very precise temperature control.